<!DOCTYPE html>
<html lang="en">
<head>
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div {
  width: 0px;
  height: 0px;
  border-top: 20px solid transparent;
  border-bottom: 20px solid red;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
}
</style>
<body>
    <div></div>
</body>
</html>